personally i would go blade, but that's your choice

knights would be a lot cheaper, and easier to lvl compared to a blade.
you need to know what one of the two classes you want to become before you start lvling past 15 (after your first job change) otherwise you'll mess up the stats. at 15 all your stats will reset, but at your second job change (mercenary to blade/knight) they don't. if you want to become a blade, all you got to do is pump STR up, everything in STR

knights are STA/STR build, but you should go onto flyff's forums on their site, and look up a few guides on them, to see what best suits you.
If you can't remember your acc/pass, and make a new one. start on a popular server, think Lawolf & mushpie are the most popular here on jsp.
This site will help a lot of questions, (Flyff wiki)
http://flyff-wiki.gpotato.com/index.php/Main_Page